Process for separating para-xylene from a mixture of C8 and C9 aromatic hydrocarbons

ABSTRACT

The process includes at least two adsorptive separation zones to separate para-xylene from a feed stream comprising C8 aromatic hydrocarbons and at least one C9 aromatic hydrocarbon component. An adsorbent comprising X or Y zeolite and a heavy desorbent are used in the first adsorptive separation zone to produce an extract stream comprising para-xylene and a raffinate stream comprising para-xylene depleted C8 aromatic hydrocarbons, the C9 aromatic hydrocarbon, and the desorbent. The raffinate stream is separated in a raffinate distillation zone to produce a stream comprising the first desorbent component and the C9 aromatic hydrocarbon which stream is further separated in a second adsorptive distillation zone to produce a stream comprising the desorbent and a C9 aromatic hydrocarbon stream.

FIELD OF THE INVENTION

The present invention pertains to a process for the separation ofpara-xylene from a mixture of C8 aromatic hydrocarbons containing atleast one C9 aromatic hydrocarbon. In particular, the process includesat least two adsorptive separation steps.

BACKGROUND OF THE INVENTION

Para-xylene is an important raw material in the chemical and fiberindustries. For example, terephthalic acid derived from para-xylene isused to produce polyester fabrics. Para-xylene is usually separated froma mixture of para-xylene and at least one other C8 aromatic hydrocarbonby either crystallization, adsorptive separation, or a combination ofthese two techniques.

U.S. Pat. No. 3,392,113 discloses a cyclic process for the separation ofa feed mixture of fluid compounds by contacting the feed with a solidsorbent, such as molecular sieves, selective for at least one compoundof said feed mixture, and thereafter passing a fluid desorbent intocontact with the sorbent to displace the resulting selectively sorbedcompound, said desorbent ordinarily containing trace quantities ofaromatic and/or oxygenate impurities which undesirably alter thekinetics, or rates of sorption and desorption of the aforesaid process,over a number of sorption-desorption cycles, the method of stabilizingthe kinetics by contacting the desorbent with a separate bed of solidsorbent, prior to utilizing the desorbent in the desorption step, toremove said impurities.

U.S. Pat. No. 5,012,038 recognizes the common use of para-diethylbenzene(p-DEB) as a desorbent for the separation of para-xylene from C8aromatic hydrocarbon mixtures. It is also known that use of p-DEB as thedesorbent limits the C9 aromatics in the feed mixture to less than about0.1 wt %. This requirement is usually met by first distilling the feedin a so-called xylene splitter column. Otherwise, the C9 aromatichydrocarbons would gradually build up in the desorbent as it is recycledin the process because C9 aromatics are difficult to separate from p-DEBby simple fractionation and the desorbent must be recycled for economicreasons.

U.S. Pat. No. 5,012,038 and other patents such as U.S. Pat. No.4,886,930; U.S. Pat. No. 5,057,643; U.S. Pat. No. 5,171,922; U.S. Pat.No. 5,177,295; and U.S. Pat. No. 5,495,061 disclose the use ofdesorbents having higher boiling points than p-DEB to separatepara-xylene from a feed mixture having a C9 aromatic hydrocarbon contentgreater than 0.1 wt %. The C9 aromatics are then separated from thehigher boiling desorbent by fractionation. However, despite the benefitsprovided by the higher boiling adsorbents, p-DEB continues to be afrequently used desorbent for the adsorptive separation of para-xylene.

SUMMARY OF THE INVENTION

The invention relates to processes for separating para-xylene from afeed stream comprising C8 aromatic hydrocarbons and at least one C9aromatic hydrocarbon component. In an embodiment, the process maycomprise contacting an adsorbent with the feed stream and a firstdesorbent stream comprising a first desorbent component in a firstadsorptive separation zone to produce an extract stream comprisingpara-xylene and a raffinate stream comprising the para-xylene depletedC8 aromatics, the C9 aromatic hydrocarbon component and the firstdesorbent component; separating the raffinate stream in a raffinatedistillation zone to produce a second desorbent stream comprising thefirst desorbent component and the C9 aromatic hydrocarbon component;separating the second desorbent stream in a second adsorptivedistillation zone to produce a C9 aromatic hydrocarbon stream and athird desorbent stream comprising the first desorbent component.

In another embodiment, the invention may comprise separating para-xylenefrom a feed stream comprising C8 aromatic hydrocarbons and at least oneC9 aromatic hydrocarbon component the process comprising:

-   (a) contacting a first adsorbent comprising a Y zeolite or an X    zeolite with the feed stream and a first desorbent stream comprising    a first desorbent component having a boiling point of at least about    150° C. in a first adsorptive separation zone to produce a first    extract stream comprising para-xylene and the first desorbent    component and a first raffinate stream comprising para-xylene    depleted C8 aromatic hydrocarbons, the C9 aromatic hydrocarbon    component, and the first desorbent component;-   (b) passing the first extract stream to an extract distillation zone    to produce a second desorbent stream comprising the first desorbent    component and a para-xylene product stream;-   (c) passing the first raffinate stream to a raffinate distillation    zone to produce a third desorbent stream comprising the first    desorbent component and the C9 aromatic hydrocarbon component and a    raffinate product stream comprising the para-xylene depleted C8    aromatic hydrocarbons; and-   (d) passing at least a portion of the third desorbent stream and a    desorbent stream comprising a second desorbent component to a second    adsorptive separation zone comprising a second adsorbent to produce    a second extract stream comprising the first desorbent component and    the second desorbent component and a second raffinate stream    comprising the C9 aromatic hydrocarbon component and the second    desorbent component.

In an embodiment, the first desorbent component is para-diethylbenzene(p-DEB). In another embodiment, the first adsorptive separation zoneoperates in a simulated moving bed mode. In a further embodiment, thefirst desorbent stream may comprise up to 25 wt % of C9 aromatichydrocarbons. Other embodiments of the present invention encompassfurther details the descriptions of which, including preferred andoptional features are hereinafter disclosed.

Thus, in one aspect the invention provides greater flexibility byenabling the adsorptive separation of a C9 aromatic hydrocarboncomponent from a desorbent component used in the adsorptive separationof para-xylene from feed mixtures comprising C8 aromatic hydrocarbonsand at least one C9 aromatic hydrocarbon. In another aspect, theinvention provides greater flexibility by enabling the adsorptiveseparation of para-xylene from the feed mixture wherein the desorbentstream may comprise up to 25 wt % C9 aromatic hydrocarbons.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified flow scheme of an embodiment of the invention.

FIG. 2 is a simplified flow scheme illustrating an embodiment of theinvention wherein the raffinate distillation zone produces three productstreams.

FIG. 3 is a simplified flow scheme of an adsorptive separation zone ofthe invention illustrating a fixed bed embodiment.

FIG. 4 is a simplified flow scheme of an adsorptive separation zone ofthe invention illustrating a simulated moving bed embodiment.

The Figures are intended to be illustrative of the present invention andare not intended to limit the scope of the invention as set forth in theclaims. The drawings are simplified schematic diagrams showing exemplaryembodiments of process flow schemes, including process zones, helpfulfor an understanding of the invention. Details of the process zones,well known in the art, such as pumps, control valves, instrumentation,heat-recovery circuits, and similar hardware which are non-essential toan understanding of the invention are not illustrated.

DETAILED DESCRIPTION OF THE INVENTION

Two adsorptive separation steps or zones are used to separatepara-xylene from a feed stream comprising C8 aromatic hydrocarbons andat least one C9 aromatic hydrocarbon component. As used herein, the term“zone” can refer to one or more equipment items and/or one or moresub-zones. Equipment items may include, for example, one or morevessels, heaters, separators, exchangers, conduits, pumps, compressors,and controllers. Additionally, an equipment item can further include oneor more zones or sub-zones.

The feed stream is a mixture comprising at least two C8 aromatichydrocarbons; para-xylene, and at least one of meta-xylene,ortho-xylene, and ethylbenzene. The feed stream also comprises at leastone C9 aromatic hydrocarbon component, such as any of the isomers ofpropylbenzene, methylethylbenzene, and trimethylbenzene. The feed streammay comprise several or all of the C8 and C9 aromatic hydrocarbons, forexample, when the feed is derived from one or more oil refiningprocesses such as catalytic reforming, stream cracking, crystallizermother liquors, transalkylation, and xylene isomerization.

The feed to be processed by this invention may contain as much as 25 wt% C9 aromatics hydrocarbons. Feed streams having at least about 0.1 wt %C9 aromatics are contemplated for use in this process. In an embodiment,the feed stream may comprise from about 0.3 wt % to about 5 wt % C9aromatic hydrocarbons. In another embodiment, the feed stream maycomprise from about 6 wt % to about 15 wt % C9 aromatics. In anembodiment, the feed stream may not contain more than about 10 ppm-massC10+ aromatic hydrocarbons.

FIG. 1 illustrates the flow scheme of an embodiment of the presentinvention. The feed stream and a desorbent stream are introduced toadsorptive separation zone 100 via feed conduit 1 and desorbent conduit3, respectively. Adsorptive separation zone 100 comprises adsorbentchamber 110 containing an adsorbent selective for para-xylene over theother C8 aromatic hydrocarbons in the feed. Adsorptive separation zone100 produces an extract stream carried by extract conduit 5 and araffinate stream carried by raffinate conduit 7. As shown in theFigures, reference numbers of the streams and the lines or conduits inwhich they flow are the same. For example, reference number 7 may beused with equal accuracy as raffinate conduit 7, raffinate line 7,raffinate stream 7, and raffinate stream carried by raffinate conduit 7.

Adsorptive separation processes are well known in the art. In briefsummary, a feed stream and desorbent stream are introduced to anadsorbent chamber which may include one or more vessels containing anadsorbent. During an adsorption step, the adsorbent contacts the feedand selectively retains a feed component or a class of feed componentsrelative to the remaining feed components. The selectively retained feedcomponent(s) are released or desorbed from the adsorbent by contactingthe adsorbent with the desorbent. Thus, the adsorptive separationprocess produces an extract stream comprising the selectively adsorbedcomponent or class of components and a raffinate stream comprising theremaining feed components that are less selectively adsorbed. Thedesorbent stream may comprise one or more desorbent components and useof multiple desorbent streams is also known in the art. The extract andraffinate streams passing from the adsorbent chamber typically alsocomprise one or more desorbent components.

A variety of adsorptive separation techniques are well known in the artincluding fixed bed such as operating in a batch or swing bed mode,moving bed, and simulated moving bed (SMB). The invention is notintended to be limited by the particular adsorptive separation techniqueor mode of operation. Additional information regarding adsorptiveseparation principles and detail are readily available, e.g.,Kirk-Othmer Encyclopedia of Chemical Technology Vol. 1, 3rd ed.,Adsorptive Separation (Liquids) pp 563-581, 1978 and Preparative andProduction Scale Chromatography edited by G. Ganetsos and P. E. Barker,1993.

As these various adsorptive separation processes operate on the samebasic chromatographic separation principles, the following discussion ofadsorbents and desorbents applies to the various adsorptive separationtechniques or modes. The functions and properties of adsorbent anddesorbents in the chromatographic separation of liquid components arewell-known, but for reference thereto, U.S. Pat. No. 4,642,397 is hereinincorporated by reference.

Adsorbents which are selective for para-xylene relative to the other C8aromatic isomers are suitable for use in adsorptive separation zone 100.X and Y zeolites are well known in the art for separating para-xylenefrom other C8 aromatic hydrocarbons. Optionally, these zeolites maycontaining FUPAC Group 1 or 2 metal ions at exchangeable cation sites.In an embodiment, the adsorbent comprises X zeolite or Y zeolite.Optionally, the adsorbent may comprise barium, potassium, or both bariumand potassium.

It is also known that crystalline aluminosilicates, i.e., zeolites, areused in adsorptive separations of various mixtures in the form ofagglomerates having high physical strength and attrition resistance.Methods for forming the crystalline powders into such agglomeratesinclude the addition of an inorganic binder, generally a clay comprisinga silicon dioxide and aluminum oxide, to the high purity zeolite powderin wet mixture. The blended clay zeolite mixture is extruded intocylindrical type pellets or formed into beads which are subsequentlycalcined in order to convert the clay to an amorphous binder ofconsiderable mechanical strength. As binders, clays of the kaolin type,water permeable organic polymers, or silica are generally used.

Desorbent stream in line or conduit 3 used in adsorptive separation zone100 may comprise one or more desorbent components. Suitable desorbentcomponents are “heavy”, i.e., they have a boiling point of at leastabout 150° C. In an embodiment, a desorbent component has a boilingpoint greater than about 160° C. In another embodiment, a desorbentcomponent has a boiling point greater than about 170° C. Examples ofdesorbent components in stream 3 suitable for use in adsorptiveseparation zone 100 include: para-diethylbenzene, diethyltoluene,tetralin, alkyl and dialkyl tetralin derivatives, indane, naphthalene,methylnaphthalene, para-dimethylnaphthalene, and mixtures thereof. In anembodiment, desorbent stream 3 comprises para-diethylbenzene (p-DEB).

In an embodiment the instant invention recognizes that desorbentintroduced to adsorptive separation zone 100 may comprise as much as 25wt % C9 aromatics hydrocarbons. In an embodiment, desorbent stream 3 maycomprise at least about 0.7 wt % C9 aromatics. In another embodiment,the C9 aromatic hydrocarbon content of the desorbent stream in line 3introduced to adsorptive separation zone 100 ranges from about 1 wt % toabout 5 wt %; in another embodiment, the range is from about 3 wt % toabout 15 wt % C9 aromatics.

In adsorptive separation zone 100, adsorption conditions will include atemperature range from about 20° C. to about 300° C. In an embodimentthe adsorption temperature will range from about 20° C. to about 250°C.; in another embodiment the range is from about 40° C. to about 200°C. The adsorption pressure is sufficient to maintain liquid phase, whichmay be from about 1 barg to about 40 barg. Desorption conditions mayinclude the same range of temperatures and pressure as used foradsorption conditions. In a fixed bed embodiment, adsorptive separationzone 100 may use vapor phase desorption conditions to minimize theamount of desorbent that remains on the adsorbent when feed is nextintroduced.

Raffinate stream in conduit 7 removed from adsorptive separation zone100 comprises a desorbent component and the less strongly adsorbed feedcomponents such as ethylbenzene, ortho-xylene, meta-xylene, and most ofthe C9 aromatics. Although there may be a small amount of para-xylenepresent, the raffinate stream C8 aromatics may be referred to aspara-xylene depleted C8 aromatics. Extract stream in conduit 5 removedfrom adsorptive separation zone 100 comprises a desorbent component andthe most strongly adsorbed feed components including para-xylene and, ifpresent, toluene and para-methylethylbenzene.

As illustrated in FIG. 1, extract stream 5 withdrawn from adsorptiveseparation zone 100 is passed to extract distillation zone 200. Extractdistillation zone 200 comprises extract distillation column 210 andproduces para-xylene product stream in line 215 and a desorbent streamremoved in conduit 220. Extract product stream 215 may comprisesubstantially all of the para-xylene in extract stream 5 from adsorptiveseparation zone 100. As used herein, the term “substantially all” canmean an amount generally of at least 90%, preferably at least 95%, andoptimally at least 99%, by weight, of a compound or class of compoundsin a stream. In an embodiment, para-xylene product stream 215 is theoverhead or light stream from extract distillation column 210 anddesorbent stream 220 is the bottoms or heavy stream from distillationcolumn 210. In an embodiment, at least a portion of desorbent stream 220removed from extract distillation zone 200 may be recycled via optionalconduit 250 to provide at least a portion of desorbent stream 3 used inadsorptive separation zone 100. Thus, a recycle conduit providing fluidcommunication from extract distillation zone 200 to adsorptiveseparation zone 100 may be the portions of lines 220, 250, and 3defining the fluid flow path between the zones. That is, here as in theremainder of the description, conduits providing fluid communication maycomprise multiple conduits or portions thereof to define a desired fluidflow path.

Those of ordinary skill in the art will understand that the process flowand connections of various zones described herein is sufficient topractice the invention. Unless otherwise stated, the exact connectionpoint within the zones is not essential to the invention. For example,it is well known in the art that a stream to a distillation zone may besent directly to the column, or the stream may first be sent to otherequipment within the zone such as heat exchangers, to adjusttemperature, and/or pumps to adjust the pressure. Likewise, streamsleaving a zone may pass directly from a distillation column or they mayfirst pass through an overhead or reboiler section before leaving thedistillation zone.

Extract distillation zone 200 may also produce additional productstreams. As illustrated in FIG. 1, a product stream lighter thanpara-xylene may be removed from extract distillation zone by optionalconduit 230. For example, this embodiment may be used when lightimpurities in extract stream 5 such as toluene are removed to enable thepara-xylene product 215 to meet desired purity specifications. Extractdistillation zone 200 may be configured and operated as well known inthe art to make three or more product streams, e.g. adding a side drawto extract column 210, using a dividing wall distillation column, and/orincluding multiple distillation columns such as optional extractfinishing distillation column 211 illustrated in FIG. 1.

Para-methylethylbenzene (p-MEB) may also be present in extract stream 5from adsorptive separation zone 100 and may be distributed in variousratios between the para-xylene 215 and desorbent 220 products of extractdistillation zone 200. Factors that impact the p-MEB distributionbetween the products include parameters such as the design and operationof the distillation column and the boiling point(s) of the desorbentcomponent(s) employed. As recognized herein, desorbent stream 3introduced to adsorptive separation zone 100 may contain up to about 25wt % C9 aromatic hydrocarbons, which may include p-MEB. As it isdesirable for economic reasons to recycle desorbent in the process,unacceptable accumulation of p-MEB in extract distillation zonedesorbent stream 220 may be managed in a number of ways.

In an embodiment, the content of p-MEB in feed stream 1 to adsorptiveseparation zone 100 may be limited such that the amount of p-MEB in feedstream 1 is not more than about 0.05 wt % of the para-xylene in feedstream 1. In an option not illustrated, a purge stream may remove aportion of desorbent containing p-MEB from line 220 and desorbent ofhigher purity may be introduced as make-up to the flow scheme. Inanother embodiment, design and operation of extract distillation column210 increases the amount of p-MEB in para-xylene product 215. Althoughit is often desired for para-xylene product 215 to contain at least 99.7wt % para-xylene, it is not always necessary to remove p-MEB frompara-xylene product 215. For example, where para-xylene product isoxidized to make terephthalic acid, oxidation of p-MEB results in thesame product. Therefore, not removing p-MEB from para-xylene product 215may actually be beneficial.

As illustrated in FIG. 1, raffinate stream 7 from adsorptive separationzone 100 is passed to raffinate distillation zone 300. Raffinatedistillation zone 300 comprises raffinate distillation column 310 andproduces raffinate product stream 315 and desorbent stream 320. In anembodiment, the overhead or light stream from raffinate distillationcolumn 310 is raffinate product stream 315 and the bottoms or heavystream from distillation column 310 is desorbent stream 320. Raffinateproduct stream 315 may comprise substantially all of the C8 aromatichydrocarbons (the para-xylene depleted C8 aromatic hydrocarbons) inraffinate stream 7 from adsorptive separation zone 100. Desorbent stream320 removed from raffinate distillation zone 300 may comprisesubstantially all the desorbent in raffinate stream 7 removed fromadsorptive separation zone 100. In an embodiment, at least a portion ofdesorbent stream 320 produced by raffinate distillation zone 300 may berecycled via optional conduit 350 to provide at least a portion ofdesorbent stream 3 introduced used in adsorptive separation zone 100.

C9 aromatic hydrocarbons have boiling points that range from about 152°C. to about 176° C. Therefore, some of the C9 aromatic hydrocarbons inraffinate stream 7 from adsorptive separation zone 100 will pass toraffinate distillation zone desorbent stream 320 if the boiling point ofthe desorbent component is not sufficiently high such as when p-DEB isthe desorbent component. Adsorptive separation zone 400 preventsunacceptable accumulation of C9 aromatic hydrocarbons in desorbentstream 3 which may be recycled to adsorptive separation zone 100.Adsorptive separation zone 400 may also be used in embodiments where thedesorbent component has a higher boiling point than p-DEB. Although itmay be possible to separate higher boiling desorbents from C9 aromaticsvia distillation, the instant invention provides an alternate route tomanage the C9 aromatic content for such desorbents that does not requirethe raffinate distillation column to provide desorbent free of C9aromatics.

The feed stream 380 to adsorptive separation zone 400 comprises at leasta portion of desorbent stream 320 from raffinate distillation zone 300,which comprises a desorbent component from the first adsorptiveseparation zone 100 and C9 aromatic hydrocarbons. Thus, conduits 320 and380 or portions thereof which provide fluid communication from raffinatedistillation zone 300 to second adsorptive separation zone 400 may alsobe described as a C9 aromatic hydrocarbon conduit. As discussed foradsorptive separation zone 100, the invention is not intended to belimited by the particular adsorptive separation technique or mode ofoperation and any of the techniques or modes previously mentioned may beemployed in adsorptive separation zone 400. Adsorptive separation zone400 also requires a desorbent stream which is provided by conduit 20. Toavoid confusion, the term “first desorbent component” will refer todesorbent used in the first adsorptive separation zone 100 while theterm “second desorbent component” will refer to desorbent introduced byconduit 20 and used as desorbent in the second adsorptive separationzone 400.

In an embodiment, adsorptive separation zone 400 adsorption conditionsmay include a temperature range from about 20° C. to about 300° C.; inanother embodiment the temperature range is from about 20° C. to about250° C.; optionally from about 40° C. to about 200° C. The adsorptionpressures are sufficient to maintain liquid phase, which may be fromabout 1 barg to about 40 barg. Desorption conditions may include thesame range of temperatures and pressures as used for adsorptionconditions. In a fixed bed embodiment, second adsorptive separation zone400 may use vapor phase desorption conditions to minimize the amount ofsecond desorbent component remaining on the adsorbent when stream 380 isintroduced to begin the next adsorption/desorption cycle.

Adsorptive separation zone 400 comprises adsorbent chamber 410containing a second adsorbent and produces an extract stream carried byconduit 420 and a raffinate stream carried by conduit 430. In anembodiment, the second adsorbent is selective for para aromatic isomersover other aromatic isomers including the C9 aromatic component. Forexample, the second adsorbent may comprise an X or a Y zeolite.Optionally, these zeolites may contain IUPAC Group 1 or 2 metal ions atexchangeable cation sites. The second adsorbent may optionally comprisebarium, potassium, or both barium and potassium. Because the firstdesorbent component is suitable for a para selective first adsorbent, itmay be selectively retained by the para selective second adsorbent overthe C9 aromatic hydrocarbon component. The second desorbent componentmay be heavy, for example, selected from the group of possible firstdesorbent components such as, para-diethylbenzene, diethyltoluene,tetralin, alkyl and dialkyl tetralin derivatives, indane, naphthalene,methylnaphthalene, and para-dimethylnaphthalene; other than the firstdesorbent component itself.

In another embodiment, the second adsorbent has selectivity for thefirst desorbent components which have molecular diameters comparable toor smaller than para-diethylbenzene (p-DEB) over the C9 aromatichydrocarbon component. For example, the second adsorbent may comprise anMFI type zeolite as classified by Structure Commission of theInternational Zeolite Association (available at web sitewww.iza-structure.org/databases). Thus, first desorbent componentssuitable for this embodiment include p-DEB, tetralin, indane,naphthalene, methylnaphthalene, para-dimethylnaphthalene. As before, thesecond desorbent component may be selected from this same group otherthan the first desorbent component itself. The second adsorbent may bethe same as the first adsorbent, or the second adsorbent may bedifferent from the first adsorbent. For either the para selective ormolecular diameter selective adsorbents, the first desorbent componentwill be discharged from second adsorptive separation zone 400 in extractstream 420 while the C9 aromatic component will be discharged inraffinate stream 430.

The second desorbent used in adsorptive separation zone 400 may compriseone or more components. For example, light desorbent components such asbenzene and toluene are suitable second desorbents and may contain smallamounts of non-aromatics, e.g. less than about 10 wt %. In anembodiment, the second desorbent component has a boiling point thatdiffers from the boiling points of the first desorbent component and C9aromatic component by at least 5° C. Use of second desorbents heavierthan the first desorbent may provide energy savings if they areseparated as discussed below in optional steps and zones. In anembodiment, the first desorbent component is p-DEB and the seconddesorbent component is benzene, toluene, tetralin, naphthalene,methylnaphthalene, or para-dimethylnaphthalene.

Raffinate stream in conduit 430 removed from adsorptive separation zone400 comprises the second desorbent component and C9 aromatic component.In an embodiment not illustrated, raffinate stream 430 is fractionatedin a distillation zone to produce a C9 aromatic product stream and astream comprising the second desorbent component which may be recycledto second adsorptive separation zone 400.

Extract stream in conduit 420 removed from adsorptive separation zone400 comprises the first desorbent component and the second desorbentcomponent. As illustrated in FIG. 1, a portion or all of extract stream420 may be passed to optional distillation zone 500 comprisingdistillation column 510 to produce the desorbent stream in conduit 550comprising the first desorbent component which is recycled to the firstadsorptive separation zone 100. Also as illustrated, a portion or all ofextract stream 420 may optionally be passed in conduit 460 to extractdistillation zone 200 wherein the second and first desorbent components(e.g. toluene and p-DEB, respectively) may be separated and recovered aspreviously described. Optionally, a portion or all of light stream 230may be recycled via conduit 270 to provide at least a portion of thesecond desorbent component stream 20 introduced to second adsorptiveseparation zone 400. In an embodiment, a first desorbent component fromat least one of extract desorbent stream 220, raffinate desorbent stream320, and second adsorptive separation zone extract stream 420 may berecycled to provide at least a portion of desorbent stream 3 used in thefirst adsorptive separation zone 100. The C9 aromatic hydrocarboncontent and other specifications of the desorbent stream 3 passed intoadsorptive separation zone 100 may be controlled by regulating the flowrate of the various streams comprising the first desorbent componentamong the various flow scheme options. In an embodiment, secondadsorptive separation zone 400 may be operated intermittently.

In an embodiment as illustrated in FIG. 2, raffinate distillation zone300 produces a third effluent stream 318. As previously discussed, threeproduct streams are readily accomplished by those of ordinary skill inthe art of distillation. Optional second raffinate distillation column311 is illustrated in FIG. 2. Raffinate product stream 315 comprisespara-xylene depleted C8 aromatic hydrocarbons and desorbent stream 320comprises the first desorbent component and C9 aromatic hydrocarbons.The third effluent stream 318 has a higher boiling point than desorbentstream 320. Thus, in this embodiment, desorbent stream 320 isintermediate raffinate product stream and stream 318 is a bottomsproduct from raffinate distillation column 310 and may be referred to asanother desorbent stream since it comprises the first desorbentcomponent. Although some portion of the C9 aromatic hydrocarbons inraffinate stream 7 from the first adsorptive separation zone 100 may bein each of streams 315, 318, and 320, the concentration of C9 aromatics(wt %) in desorbent stream 318 is less than the concentration of C9aromatics (wt %) in desorbent stream 320. In the embodiment illustratedin FIG. 2, at least a portion of the more highly concentrated C9aromatics is passed through conduits 320 and 380 to be separated fromthe first desorbent component in second adsorptive separation zone 400.At least a portion of desorbent stream 318 having the lowerconcentration of C9 aromatic hydrocarbons is recycled to form a portionof desorbent stream 3.

As the invention is not limited by the type or mode of adsorptiveseparation, those of ordinary skill in the art can readily apply thefollowing descriptions to either adsorptive separation zone though theyare described only once. In a batch mode embodiment, an adsorptiveseparation zone comprises an adsorbent chamber having one or morevessels containing adsorbent in one or more beds. Batch mode operationconsists of sequentially introducing feed then desorbent into theadsorbent chamber. The adsorbent is thus subjected to alternateadsorption and desorption steps that produce a raffinate stream and anextract stream which alternately flow out of the adsorbent chamber. Inan embodiment, second adsorptive separation zone 400 may operate in abatch mode as illustrated in FIG. 3. Raffinate distillation zonedesorbent introduced via conduit 380 is the second adsorptive zone feedand the second adsorptive zone desorbent comprising a second desorbentcomponent is introduced in conduit 20. Thus, conduits 380 and 20 arealternately active in providing fluid communication to adsorptiveseparation zone 400. Likewise, the raffinate 430 and extract 420conduits are alternately active in providing fluid communication of theraffinate and extract streams, respectively, from adsorptive separationzone 400. As shown the streams may enter or exit the adsorbent chamberthrough individual inlets or a common inlet with valves, not shown,controlling the flows as is commonly known.

In swing bed mode, the adsorbent chamber comprises at least twoadsorbent beds or vessels each of which is operated in batch modewherein the adsorbent beds may be operating at different steps of theadsorption/desorption cycle. Swing bed mode may approach continuousproduction when the adsorbent chamber includes sufficient vesselsoperating at different points in time of the adsorption/desorption cycleto provide more uniform product quality from the overall adsorptiveseparation zone. Both the batch mode and swing bed modes are types offixed bed adsorptive separation processes. In fixed bed adsorptiveseparations, desorption conditions may be similar to the adsorptionconditions. In another embodiment, vapor phase desorption conditions maybe used to minimize the amount of desorbent remaining on the adsorbentwhen feed is introduced to begin the next adsorption/desorption cycle.For example, the desorption pressure may be decreased and/or thetemperature may be increased relative to the adsorption conditions. Inan embodiment, at least one of first adsorptive separation zone 100 andsecond adsorptive separation zone 400 is a fixed bed adsorptiveseparation zone and either or both of zones 100 and 400 may operate inbatch or swing bed mode.

Either or both adsorptive separation zones may also operate as a movingbed adsorptive separation system wherein adsorbent moves through theadsorbent chamber while the feed and desorbent streams are introduced toand the extract and raffinate streams are withdrawn from the adsorbentchamber at separate fixed locations.

In an embodiment, at least one of first adsorptive separation zone 100and second adsorptive separation zone 400 is a simulated moving bed(SMB) adsorptive separation zone. In another embodiment, the firstadsorptive separation zone 100 is a simulated moving bed adsorptiveseparation zone and the second adsorptive separation zone 400 is a fixedbed adsorptive separation zone.

FIG. 4, illustrates an embodiment wherein adsorptive separation zone 100operates as a simulated moving bed (SMB) comprising an adsorbent chamber110 having at least eight transfer points 115, a fluid distributor 120,and at least one transfer line 125 providing fluid communication betweeneach transfer point and the fluid distributor. The adsorbent chamber 110contains a number of separate beds 112 of an adsorbent selective forpare-xylene. Each bed is in fluid communication with one of the transferpoints. In an embodiment the adsorbent chamber has 16 transfer points.In another embodiment the adsorbent chamber comprises two vesselsconnected in series, each vessel having 12 transfer points.

In the SMB embodiment, four primary process streams: the feed,desorbent, extract, and raffinate streams are passed simultaneously intoand out of the adsorptive separation zone as the adsorption anddesorption steps are carried out simultaneously. Feed conduit 1 anddesorbent conduit 3 provide fluid communication to fluid distributor120. The raffinate conduit 7 and extract conduit 5 provide fluidcommunication from fluid distributor 120. The fluid distributor directsthe process streams to and from the adsorbent chamber 110 via transferlines 125 and transfer points 115. At least four of the transferline/transfer point pairs are active at a given time. That is, each ofthe four primary process streams flows through one transfer line/pointpair. Additional transfer line/point pairs may also be active whenoptional streams flow to or from the adsorbent chamber. Examples ofoptional streams are given in U.S. Pat. No. 3,201,491 and U.S. Pat. No.4,319,929.

The fluid distributor 120 and an associated controller, not shown,increment the location of the active transfer lines/points periodicallyalong the adsorbent chamber to the next transfer point to simulatemovement of the adsorbent in the opposite direction of the transferpoint movement. In an embodiment, the locations of the active transferpoints are shifted down the adsorbent chamber to simulate upwardmovement of the adsorbent, and the fluid phase is circulated through theadsorbent chamber in a downward direction. Although not shown in thedrawing, the first and last beds in the adsorbent chamber are connectedvia a conduit and pump to ensure continuous fluid flow in the desireddirection. The operating steps, principles, and equipment used in SMBadsorptive separations are well known in the art. U.S. Pat. No.2,985,589; U.S. Pat. No. 3,310,486; and U.S. Pat. No. 3,686,342 areherein incorporated by reference for their teachings with respect to SMBadsorptive separations.

In SMB adsorptive separation processes, the steps or operational zonesin the adsorbent chamber are defined by the position of the input andoutput streams as follows. Zone 1, the adsorption zone, includes theadsorbent between the feed inlet and raffinate outlet. Zone 2, thepurification zone, includes the adsorbent between the feed inlet and theextract outlet and is located upstream of Zone 1. Zone 3, the desorptionzone, includes the adsorbent between the extract outlet and thedesorbent inlet and is located upstream of Zone 2. Optional Zone 4, abuffer zone, where used includes the adsorbent between the desorbentinlet and the raffinate outlet. Further details on equipment andtechniques in an SMB process may be found, for example, in U.S. Pat. No.3,208,833; U.S. Pat. No. 3,214,247; U.S. Pat. No. 3,392,113; U.S. Pat.No. 3,455,815; U.S. Pat. No. 3,523,762; U.S. Pat. No. 3,617,504; U.S.Pat. No. 4,133,842; and U.S. Pat. No. 4,434,051.

The fluid distributor 120 may be a rotary valve type as described inU.S. Pat. No. 3,040,777; U.S. Pat. No. 3,422,848; and U.S. Pat. No.4,409,033 or a manifold/multivalve type system as in U.S. Pat. No.4,434,051. Co-current SMB operations as described in U.S. Pat. No.4,402,832 and U.S. Pat. No. 4,498,991 may also be used. Equipmentutilizing these principles is familiar, in sizes ranging from pilotplant scale as in U.S. Pat. No. 3,706,812 to commercial scale havingflow rates from a few cc per hour to many thousands of gallons per hour.The invention may also be practiced in a co-current, pulsed batchprocess, like that described in U.S. Pat. No. 4,159,284 or in aco-current, pulsed continuous process, like that disclosed in U.S. Pat.Nos. 4,402,832 and 4,478,721.

1. A process for separating para-xylene from a feed stream comprising C8aromatic hydrocarbons and at least one C9 aromatic hydrocarboncomponent, the process comprising: (a) contacting a first adsorbentcomprising a Y zeolite or an X zeolite with the feed stream and a firstdesorbent stream comprising a first desorbent component having a boilingpoint of at least about 150° C. in a first adsorptive separation zone toproduce a first extract stream comprising para-xylene and the firstdesorbent component and a first raffinate stream comprising para-xylenedepleted C8 aromatic hydrocarbons, the C9 aromatic hydrocarboncomponent, and the first desorbent component; (b) passing the firstextract stream to a first extract distillation zone to produce a seconddesorbent stream comprising the first desorbent component and apara-xylene product stream; (c) passing the first raffinate stream to araffinate distillation zone to produce a third desorbent streamcomprising the first desorbent component and the C9 aromatic hydrocarboncomponent and a raffinate product stream comprising the para-xylenedepleted C8 aromatic hydrocarbons; and (d) passing at least a portion ofthe third desorbent stream and a stream comprising a second desorbentcomponent to a second adsorptive separation zone comprising a secondadsorbent to produce a second extract stream comprising the firstdesorbent component and the second desorbent component and a secondraffinate stream comprising the C9 aromatic hydrocarbon component andthe second desorbent component.
 2. The process of claim 1 wherein thefirst desorbent component is selected from the group consisting ofpara-diethylbenzene, diethyltoluene, tetralin, tetralin derivatives,indane, naphthalene, methylnaphthalene, and para-dimethylnaphthalene. 3.The process of claim 1 wherein the raffinate distillation zone furtherproduces a fourth desorbent stream, the fourth desorbent stream having alower wt % concentration of C9 aromatic hydrocarbons than the wt %concentration of C9 aromatic hydrocarbons of the third desorbent streamand recycling at least a portion of the fourth desorbent stream to step(a) as at least a portion of the first desorbent stream.
 4. The processof claim 1 further comprising passing the second extract stream to asecond extract distillation zone to produce a fifth desorbent streamcomprising the first desorbent component and recycling at least aportion of the fifth desorbent stream to step (a) as at least a portionof the first desorbent stream.
 5. The process of claim 1 furthercomprising recycling at least a portion of the second desorbent streamto step (a) as at least a portion of the first desorbent stream.
 6. Theprocess of claim 1 further comprising recycling a portion of the thirddesorbent stream to step (a) as at least a portion of the firstdesorbent stream.
 7. The process of claim 1 further comprising recyclingat least a portion of the second extract stream to the first extractdistillation zone.
 8. The process of claim 1 wherein the secondadsorbent comprises an MFI type zeolite.
 9. The process of claim 1wherein the second adsorbent comprises a Y zeolite or an X zeolite. 10.The process of claim 9 wherein the first desorbent component ispara-diethylbenzene.
 11. The process of claim 1 wherein the feed streamcomprises no more than about 25 wt % C9 aromatic hydrocarbons.
 12. Theprocess of claim 1 wherein the first desorbent stream comprises no morethan about 25 wt % C9 aromatic hydrocarbons.
 13. The process of claim 1wherein the feed stream further comprises para-methylethylbenzene in anamount not more than about 0.05 wt % of the para-xylene in the feedstream.
 14. The process of claim 1 wherein the feed stream furthercomprises no more than about 10 ppm-mass of C10+ aromatic hydrocarbons.15. The process of claim 1 wherein the first adsorbent zeolite furthercomprises barium.
 16. The process of claim 1 wherein the firstadsorptive separation zone is a simulated moving bed adsorptiveseparation zone.
 17. The process of claim 16 wherein the simulatedmoving bed adsorptive separation zone operates in counter-current modeat a temperature ranging from about 20° C. to about 300° C. and apressure ranging from about 1 barg to about 40 barg.
 18. A process forseparating para-xylene from a feed stream comprising C8 aromatichydrocarbons and C9 aromatic hydrocarbons includingpara-methylethylbenzene, the process comprising: (a) contacting a firstadsorbent comprising a Y zeolite or an X zeolite with the feed streamand a first desorbent stream comprising para-diethylbenzene in asimulated moving bed adsorptive separation zone to produce a firstextract stream comprising para-xylene, para-methylethylbenzene, andpara-diethylbenzene and a first raffinate stream comprising para-xylenedepleted C8 aromatic hydrocarbons, para-methylethylbenzene depleted C9aromatic hydrocarbons, and para-diethylbenzene; (b) passing the firstextract stream to a first extract distillation zone to produce a seconddesorbent stream comprising para-diethylbenzene and a para-xyleneproduct stream; (c) passing the first raffinate stream to a raffinatedistillation zone to produce a third desorbent stream comprisingpara-diethylbenzene and the para-methylethylbenzene depleted C9 aromatichydrocarbons and a raffinate product stream comprising the para-xylenedepleted C8 aromatic hydrocarbons; and (d) passing at least a portion ofthe third desorbent stream and a stream comprising a second desorbentcomponent to a fixed bed adsorptive separation zone comprising a secondadsorbent to produce a second extract stream comprisingpara-diethylbenzene and the second desorbent component and a secondraffinate stream comprising C9 aromatic hydrocarbons and the seconddesorbent component.
 19. The process of claim 18 wherein the secondadsorbent comprises a zeolite selected from the group consisting of a Yzeolite, an X zeolite and an MFI type zeolite; and the second desorbentcomponent is selected from the group consisting of benzene, toluene,tetralin, naphthalene, methylnaphthalene, and para-dimethylnaphthalene.20. The process of claim 19 further comprising recycling at least aportion of the second extract stream to the first extract distillationzone.